Primary Treatment of Waste Water | Civil Engineering SSC JE (Technical) - Civil Engineering (CE) PDF Download

Chapter 5

 PRIMARY TREATMENT OF WASTE WATER

Preliminary  treatment: It  consists  of  separating  the  floating  materials  and  also  heavy inorganic solids. It also helps in removing the oils and greases from sewage. The processes used  are  screenings  for  removing  floating  materials.
Grit  chambers  for  removing  grit, skimming tanks to remove oil and grease.
Preliminary treatment: It consists of removing large suspended organic material. This is done is sedimentation tanks. Sedimentation tank principles are same as we discussed in drinking water treatment.
Secondary treatment: The secondary treatment is also called biological treatment is carried out by changing the character of the organic matter and converting it into stable forms.

There are two types of treatments are present 1. Filters    2. Activated sludge process

1. FILTERS:
a) contact beds: used at very small paints b) Intermittent sand filters:  used at small plants c) Trickling filters.

Trickling filters: -These are also called as percolating filters or sprinkling filters, consist of tanks of coarser filtering media over which the sewage is allowed to sprinkle or trickle down by means of spray nozzles or rotary distributions.

The percolating sewage is collected at the bottom of the tank through a well-designed under-drainage system. Microorganisms and bacteria which are present in sewage, get attached to the filter media. The organic matter is degraded by the aerobic bacteria.
Efficiency of trickling filter h% =

Where m  =organic loading kg/ha-m/day High rate trickling filters: Re-circulation of sewage is an essential and important feature of high rate trickling filters Re-circulation factor F=

 where R/I is ratio of the volume of sewage recirculated R to the volume of raw sewage I.
(R/I)=Recirculation ratio Efficiency
h% =

Where Y=Total organic loading in kg/day
V=Filter volume.
 = u  (organic loading).

Final efficiency of two stage filter:

where  = Y' =   ToatlBOD in effluent from first stage in kg/day
V' =   Volume of second stage filter in ha-m
F' =   Recirculation factor for second stage

 2. ACTIVATED SLUDGE PROCESS:
The sewage effluent from primary sedimentation tank is mixed with 20 – 30% of own volume of activated sludge which contains a large concentration of highly active aerobic microorganisms.

The mixer enters aeration tank where the microorganisms coated around sludge solids and the sewage are intimately mixed together with a large quantity of air for about 4-8 hours. Under these conditions the moving organisms will oxidize the organic matter and suspended, colloidal matter tend to coagulate and form a precipitate which settles down readily in the secondary settling tank. The settled sludge containing microorganisms called activated sludge is continuously being produced by this process and a portion of it being utilized and sent back to the aeration tank whereas the excess portion is disposed off properly along with the sludge collected during primary treatment after digestion.

The volume of returned activated sludge depends upon the extend of BOD desired to be removed. It is expressed as percentage of flow of sewage as 
(Q/RQ) 100
´ . Where  Q_R is the returned slude rate in m³/day and Q is the sewage inflow rate in   m³/day.
Aeration period (Hydraulic Retention time) HRT:

HRT=

BOD loading per unit volume of Aeration tank (volumetric loading):

Volumetric  BOD  loading = organic loading  =

Food (F) to micro - organism (M) ratio (F/M)

:

Sludge age: The average time for which particles of suspended solids remain under aeration is called sludge age. This  time is also called solids detention time or mean cell residence time (MCRT).
Mass of suspended solids in system SludegeAge=

Slude volume index (SVI): Volume occupied in ml by one gm of solids in the mixed  liquor after settling of for 30 min.

sludge circulation Rate : Q_R. where X_t = MLSS in mg/I

Sludge circulation ratio

= MLSS in returned sludege mg/l.

Tertiary treatment of wastewater is any process that occurs after secondary treatment. It can be polishing processes that improve suspended solids removal or nutrient removal processes. Nutrient  removal  includes processes  like  nitrification/denitrification,  ammonia  stripping, phosphorous precipitation, and land application or overland flow.

 Effluent polishing
Effluent  polishing  is  a  physical  treatment  process.  Effluent  polishing  is  normally accomplished filtration of secondary effluent in an effort to remove suspended solids from ashing or pin floc problems. These filters are similar to those used to treat drinking water.

Rapid Sand Filters Conventional rapid sand and mixed media filters have many design similarities. The basic components  of the  filters  include  all  of  the  components  described  below.  The  main differences will be in the type of media that is used and the valving configurations.

Filter boxes may be constructed as rectangles, squares, round, or as the outer segment of a ring. A filter box is approximately ten feet deep, though its surface dimensions may vary depending on the volume of water to be filtered.

The underdrain serves three basic functions. Although it supports the filter media and collects the filtered water, its most important function is to evenly distribute the backwash water throughout the filter. Leopold tile and Wheeler blocks are two popular types of underdrain systems.

Effluent polishing ponds Effluent polishing ponds are shallow aerobic lagoons that receive treated secondary effluent. They are sometimes mechanically aerated. The non-aerated ponds will stay aerobic because there shouldn't be much BOD left to create an oxygen demand. Usually wind action and surface oxygen transfer will provide adequate aeration since the ponds are only 2-3 feet deep. They provide a final chance to remove suspended solids and lower the effluent BOD. They may also provide the detention time and U-V radiation from sunlight to naturally dechlorinate the effluent after disinfection.

Nutrient Removal The first goal of a wastewater treatment is to remove suspended solids and BOD. Suspended solids created sediment in the receiving waters and organics will continue to decompose, using up oxygen that the aquatic wildlife needs. There is another problem that affects aquatic wildlife  and  directly  impacts  the water  quality  of  the  receiving  waters.  Nitrogen  and phosphorous compounds in the wastewater effluent can be toxic to fish, as is the case with ammonia, and can act as natural fertilizers that increase the growth rate of aquatic plants like algae. This can result in algae blooms that can choke out other aquatic life in the lake or river. The nitrogen cycle that occurs, as ammonia is oxidized to nitrates, also results in a reduction of dissolved oxygen (DO) in the receiving waters. The first treatment processes that dealt with the nitrogen issue were nitrification processes. The intent was to convert the ammonia present in the secondary processes to nitrates. This would create a more stable form of nitrogen and minimize the oxygen depletion that would occur in the river. But the nitrates still acted as an aquatic fertilizer that created algae problems. Denitrification processes were developed to convert nitrates to elemental nitrogen gas that can be stripped from the effluent by aeration.

Nitrification
Nitrification of wastewater will occur after most of the BOD has been removed. If enough dissolved oxygen is available, nitrifying bacteria like Nitrobacteria and Nitrosomonas will begin oxidizing ammonia (NH3) into nitrites (NO2) first and then nitrates (NO3).
This process requires a tremendous amount of oxygen. Dissolved oxygen levels need to be in the  4-6  mg/ L  range  to  accomplish  nitrification.  Alkalinity  is  also  removed  during  this process. Nitrification usually occurs in the latter stages of multi-staged activated sludge systems and extended aeration systems. The long detention times give the bugs time to oxidize the BOD and then oxidize the ammonia to nitrates. Multistaged RBC processes can also nitrify if they are aerated to maintain the higher DO levels.

Denitrification The  most  common  process  used  to  remove  the  nitrogen  completely  is  known  as denitrification. It follows the nitrification process. It utilizes denitrifying bacteria to remove the oxygen from the nitrate compounds. Nitrates are converted into nitrogen gas (N2), which effectively removes the nitrogen from the waste flow.

Phosphorus removal
Phosphorus removal is also a chemical process. Phosphorous can be precipitated as a floc particle using the same type of process that surface water systems use for softening drinking water. Alum or lime can be used as the coagulant. The treatment equipment will include a tertiary flocculation and sedimentation process and effluent filtration.